The present invention relates to data communications equipment, e.g., modems, and, more particularly, to Mu-law modems.
A "Mu-law modem" is identical to an analog modem with the exception that the Mu-law modem does not have an analog interface to the public switched telephone network (PSTN). Instead, the Mu-law modem couples to the PSTN via a digital interface. Typically, an end user utilizes a Mu-law modem behind a customer-premises private branch exchange (PBX), which itself is connected to the PSTN via wideband digital facilities like T1, etc. In such an arrangement, the Mu-law modem can be coupled to the PBX over in-house wiring or the modem can be physically resident in the PBX itself.
Whether coupled to the PBX or physically in the PBX, the Mu-law modem generates a 64 thousand bit per second (kbps) DS0 data stream for transmission through the PSTN to a far-end, or remote, data endpoint of a data connection. As known in the art, this DS0 data stream is a sequence of pulse-code modulated (PCM) samples using the same analog-to-digital sampling and encoding technique used by the PSTN for transmission of voice-band signals through the PSTN. Specifically, the digital signal processor of the Mu-law modem uses either of the standard companding encoding schemes, Mu-law or A-law, as defined in CCITT Recommendation G.711, to produce the DS0 data stream. This DS0 data stream transits the PSTN exactly like those DS0 streams created at the central office/local loop interface of the PSTN.
The obvious benefits of using a Mu-law modem are (1) superior performance since echoes and other impairments are eliminated from one end of the data connection and (2) lower cost, smaller size, etc. due to the elimination of the analog interface from the modem.
In addition, another benefit of using a Mu-law modem is the possibility of data transmission at speeds approaching the DS0 data rate. Unfortunately, this requires a completely digital connection between the two data endpoints--which cannot be guaranteed. For example, the PSTN itself might include an analog facility as part of the data connection. In addition, the other data endpoint is typically coupled to the PSTN via an analog local loop, which results in the DS0 data stream being converted back to a voice-band analog signal. As a result, the actual data rate of the data connection is limited to standard analog-based data transmission rates.
However, with the increased deployment of all-digital facilities both in the PSTN and into customer premises, it can be expected that there will be an increasing frequency of data connections in which there are no analog links between the data endpoints. These "Mu-law modem to Mu-law modem" data connections offer the opportunity to greatly increase the effective data transmission rate by taking advantage of this all-digital connection.
Unfortunately, the PSTN does not provide notification to a Mu-law modem when the data connection is entirely digital. As a result, a Mu-law modem cannot determine when an all-digital connection exists. Consequently, a Mu-law modem is limited to standard analog-based data transmission unless a priori a particular data connection is known to be all-digital. Such a situation might exist in a dedicated "point-to-point" data connection. In this instance, the Mu-law modem can be manually administered to a digital mode of operation.
Indeed, even if the modem endpoints could identify themselves to each other, this still does not solve the problem of whether the connection is all-digital. For example, U.S. Pat. No. 5,311,578, issued May 10, 1994, to Bremer et al., entitled "Technique for Automatic Identification of a Remote Modem," describes the use of a "low-level" identification signal within an industry standard answer tone to identify the answering modem. This technique would allow a Mu-law modem to recognize what type of modem was at the opposite end--but identifying the modem at the opposite end does not guarantee that intervening data connection is completely digital.